# The Reconstructive Austenite-Ferrite Transformation in Low-Alloy Steels

## R. C. Reed and H. K. D. H. Bhadeshia

## Abstract

A thermodynamic model has been coupled with simplified kinetic
theory, so that, subject to a number of assumptions, the one-
dimensional parabolic thickening constant alpha(1) for
allotriomorphic ferrite growing from austenite can be estimated
as a function of temperature and composition. To do this,
kinetic theory for ternary Fe-C-X systems (where X represents a
substantial alloying element) is extended to multicomponent
alloys. Values of alpha(1) calculated assuming local
equilibrium and paraequilibrium are compared. Consistent with
recent calculations, the slope of the alpha(1) versus
temperature plot is found to change abruptly on entry into the
negligible partitioning local equilibrium regime, consistent
with an increase in interfacial velocity. At very high
supersaturations, the effect of the cross-terms in the
diffusivity matrix appears to be small and only then can their
effect be ignored. At temperatures below the Ae3', the value of
alpha(1), calculated assuming local equilibrium, is less than
that calculated assuming paraequilibrium. Classical nucleation
theory is used to model the ferrite allotriomorphs as discs
growing from prior austenite grain boundaries. It has been
demonstrated that the model developed here can reproduce the C-
curve behaviour typical of those parts of the time-temperature-
transformation diagrams that are due to allotriomorphic
ferrite, provided the paraequilibrium mode of transformation is
assumed to be operative. This work therefore suggests that in
multicomponent alloys, the state of true local equilibrium does
not exist at the advancing interface. Some problems associated
with the paraequilibrium mode of transformation during
reconstructive growth are discussed.
*
Materials Science and Technology,* Vol. 8, 1992, 421-435.

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